1. Technical Field
The present invention relates to an electrophoretic display sheet and a manufacturing method therefor.
2. Related Art
Electrophoretic display sheets that utilize electrophoresis of particles are commonly used to form image display units of electronic paper, for example. The electrophoretic display sheets are excellent in terms of flexibility and power saving and are particularly suitable as image display units of electronic paper.
An electrophoretic display sheet includes a pair of electrodes disposed opposite each other and a display layer disposed therebetween. The display layer is filled with a dispersion liquid in which, for example, white particles to be positively charged and black particles to be negatively charged are dispersed in a liquid phase dispersion medium. Such an electrophoretic display sheet is configured such that a voltage is applied between the pair of electrodes to cause the white particles and black particles to migrate in an intended direction so as to display a desirable image.
With regard to the configuration of an electrophoretic display sheet, a configuration using a microcapsule in which a dispersion medium including electrophoretic particles is enclosed (hereinafter referred to as an MC method) and a configuration of filling each region divided by a partition with a similar dispersion medium, as in JP-A-2007-65288, (hereinafter referred to as a partition method) are known.
The MC method has, for example, the following advantages. Electrophoretic particles are confined within capsule shells, and therefore less segregation and less sedimentation occur. A binder resin is provided around the capsule shells, and therefore adhesiveness between substrates is obtained. The aperture ratio, which affects display, can be made large. In contrast, the method raises, for example, the following issues. The capsule shells and the binder resin decrease the transparency of the sheet. The display performance is reduced because of a decrease in the space over which electrophoretic particles can migrate. The capsule shells are easily deformed under pressure applied by pushing a display surface, for example, during an input operation on a touch panel.
On the other hand, the partition method has advantages in that, for example, the space over which electrophoretic particles can migrate is large, the partition structure gives resistance to pressure imposed on the display surface, and the high transparency results in high display performance. In addition to these advantages, JP-A-2007-65288 discloses that electrophoretic particles are prevented from condensation by optimizing the height of a partition and thus excellent display characteristics can be achieved.
With the partition method of the electrophoretic display sheet disclosed in JP-A-2007-65288, however, there are problems in that a high sheet strength is hard to obtain, and it is difficult to secure sufficient display quality and reliability.
In particular, as illustrated in FIG. 11A, a pair of a first substrate 10 and a second substrate 20 constituting an existing electrophoretic display sheet 90 are arranged in such a manner as to be basically affixed at their peripheral edge portions by using only a sealant 60, and therefore it is difficult to secure sufficient practical strength. Furthermore, a gap (flow path) g is secured between the crest of a partition 140 formed on the first substrate 10 and the second substrate 20 (a common electrode 21) such that the partition 140 does not completely separate an electrophoretic dispersion liquid. The segregation and sedimentation of electrophoretic particles are likely to occur.
These phenomena, as illustrated in FIG. 11B, become marked particularly in the case of using a flexible substrate that is deformable. There are few portions where the two substrates opposite each other are in contact with each other, and therefore it is difficult to secure sufficient sheet strength. Further, the gap g between the partition 140 and the second substrate 20 is large, which might cause an electrophoretic dispersion liquid to leak. Thus, it is difficult to secure intended display quality, resulting in poor reliability.